(1) Field of the Invention
The present invention relates to a supervisory controlling technique of an optical repeater to be applied to various optical communications systems, and particularly to a supervisory controlling method and a supervisory controlling system for transmitting supervisory control information making use of the Raman effect.
(2) Related Art
In various optical communications systems, it has been desired to further increase capacities of the systems due to increased data communication traffics such as in the Internet. As a method to realize increased capacities of optical communications systems, attention has been directed to a WDM optical transmission system for transmitting wavelength-division multiplexed (WDM) signal light including a plurality of optical signals of different wavelengths. Further, there has been also advanced development of an optical amplifying-and-repeating transmission system for amplifying a signal light by optical repeaters adopting optical amplifiers so as to perform the repeating transmission of the signal light.
In the conventional optical amplifying-and-repeating transmission system, as an optical amplifier used in optical repeater, there has been utilized an optical fiber amplifier that supplies an excitation light to such as an optical fiber doped with rare earth element to thereby amplify a signal light. Specifically, as an optical amplifier of optical repeater, there has been widely used an optical fiber amplifier (EDFA) that utilizes an optical fiber doped with Erbium as a rare earth element.
For the optical repeater utilizing such an EDFA, there is known a method to perform a supervisory control of operating conditions and the like, in accordance with supervisory control information to be transmitted together with a signal light between a transmission side terminal device and a reception side terminal device in a system. In a conventional supervisory controlling method of the optical repeater, as a specific method to transmit a signal light accompanied by supervisory control information toward the reception side terminal device from the optical repeater, for EDFA of an optical repeater, there has been proposed a method, as shown in (A) of FIG. 11, for supplying an excitation light which is modulated in accordance with information about supervisory control to an erbium doped fiber (EDF), to thereby apply the modulation of several percents onto the whole of signal light Ls. Alternatively, for example, as shown in (B) of FIG. 11, there has been proposed a method to provide a dedicated channel for supervisory control at an end portion of an amplification band of EDFA and to multiplex a supervisory control signal light Lsv and a signal light Ls.
However, for the conventional supervisory controlling method as mentioned above, in case of transmitting the supervisory control information by applying the modulation onto the excitation light for the EDFA, there is a possibility to cause an increase of penalty in receiving and processing the signal lights at the terminal device due to the modulated components superimposed onto the signal lights, respectively. Further, there is a problem that the transmission characteristics of the WDM signal light are fluctuated depending on wavelengths, because it is difficult to equally apply the modulation onto the optical signals of all the wavelengths included in the WDM signal light, respectively. On the other hand, in case of transmitting the supervisory control signal light by providing the dedicated channel, since at least one of waves within the amplification band of the optical amplifier is occupied for the supervisory control, there is defectively caused constraints on the effective usage of the limited amplification band of the optical amplifier, thereby resulting in a narrowed wavelength band for the signal lights.
Meanwhile, there have been investigated optical repeaters for amplifying and repeatingly transmitting signal lights by utilizing the Raman effect, such as by virtue of the recent development of optical fibers or high power light sources. The light amplification (Raman amplification) using the Raman effect utilizes such a phenomenon that, when a strong light (excitation light) is propagated through an amplifying medium such as an optical fiber, there is caused a Raman scattering phenomenon in which the thus input light is converted into a light (a Stokes light) of a longer wavelength, and further, if there exists a light (light to be amplified) coincident with the wavelength range where the Stokes light is generated, the wavelength conversion becomes frequent to thereby amplify the light to be amplified (stimulated Raman scattering). Such Raman amplification has an advantage capable of forming an amplification band in an arbitrary wavelength range, and has a wide amplification range. Thus, it is expected to apply those optical repeaters utilizing the Raman amplification to optical amplifying-and-repeating transmission systems, so as to enable to further widen the bands of the optical repeaters utilizing such as the aforementioned conventional EDFA's.
However, even in such optical repeaters utilizing the Raman amplification, it is also assumed that the aforementioned supervisory control is performed, thereby causing the same problem as in the above, if the conventional supervisory controlling method is applied in the same manner as the above. To solve such a problem concerning the supervisory control, it is demanded to realize a novel supervisory controlling technique taking account of the features of the Raman amplification.